Elements for steering the beam of helical antenna for use in a borehole penetrating an earth formation

ABSTRACT

Active control elements for switching the radiation pattern of the helical antenna between the pancake beam and bidirectional pencil beam for use in mapping an earth formation penetrated by a borehole.

United States Patent Kelgo lizuka Toronto, Ontario, Canada 876,533

Nov. 13, 1969 June 8, 1971 Chevron Research Company San Francisco,Calif.

Inventor App]. No Filed Patented Assignee ELEMENTS FOR STEERING THE BEAMOF HELICAL ANTENNA FOR USE IN A BOREHOLE PENETRATING AN EARTH FORMATION15 Claims, 16 Drawing Figs.

US. Cl 324/5, 324/6, 324/7 Int. Cl G0lv 3/12, GOlv 3/18 [50] Field ofSearch 324/6, 5, 7

[56] References Cited UNITED STATES PATENTS 3,449,657 6/1969 Fredriksson324/6 Primary ExaminerGerard R. Strecker Alt0meys-A. L. Snow, F. E.Johnston, G. F. Magdeburger, R.

L. Freeland, Jr. and H. D. Messner ABSTRACT: Active control elements forswitching the radiation pattern of the helical antenna between thepancake beam and bidirectional pencil beam for use in mapping an earthformation penetrated by a borehole.

PATENTED JUN 8197i SHEET 1 UF 4 INVENTOR 7652 M4 123M419. V AT TO R N5V5 PATENTED JUN 8 1971 SHEET 2 [IF 4 I 4A.: ATTORNEYS INVENTOR V [/60I} ELEMENTS FOR STEERING THE BEAM OF HELICAL ANTENNA FOR USE IN ABOREHOLE PENETRATING AN EARTH FORMATION This invention relates to theexploration for oil and to the mapping of the sides of a salt body fromwithin a borehole penetrating that body. More particularly, theinvention relates to a method and apparatus for sequentially emittingelectromagnetic energy from an antenna system within the well bore intothe salt dome at a known elevation and receiving reflections of thelaunched energy from the sides of the dome. The transmission, reflectionand reception of the energy are then related to time; and the time oftravel of the emitted energy (from the source to the reflector and back)is related to horizontal distance and recorded in accordance with thedepth of the antenna below the earths surface to permit mapping ofborehole). interface of the salt dome.

A particular object of the present invention is to provide a novelmethod and apparatus for controlling the radiation pattern of theantenna, thus facilitating the mapping of the boundary of the salt dome.In accordance with the present invention, the radiation pattern of theantenna can be altered between (a) omnidirectional and (b) bidirectionalin azimuth with respect to the antenna axis (and, assuming the antennais coincident with the borehole, the axis of the borehold). In theelevational direction, i.e., in planes perpendicular to the axis of theborehole, the radiating fields (in either operating mode) are highlydirectional. By comparing the radar images from reflections derived bythe antenna operated in both operating modes, prospecting for oil and/orother minerals is considerably facilitated.

The art to which the present invention relates is described in U.S. Pat.No. 3,449,657, issued June 10, 1969, entitled Helical Antenna forlrradiating an Earth Formation Penetrated by a Borehole and Method ofUsing Same," O. A. Fredriksson et al. In that patent a method andapparatus is described in which a helical antenna is utilized formapping the distance to the flanks of a salt dome from a boreholepenetrating the dome. In that patent a helical antenna system isdescribed to accomplish emission of electromagnetic radiation in theenvironment of a borehole drilled thousands of feet into a salt dome.The radiated energy is transmitted, in the transmitting mode, from thehelical antenna into the dome. Reflected energy from electromagneticdiscontinuities at the flanks of the dome (echoes) is received by theantenna, in the receiving mode. Travel time of the energy from thediscontinuity and back is measured and the distance from the antenna tothe near salt flank is approximated by computation from the knownvelocity of energy transmission through the salt to permit mapping ofthe dome.

In the aforementioned patent, the efficiency of the antenna is improvedby forming the antenna radiating elements of a multiplicity ofcoextensive pairs of interwound helical conductors (say first and secondcoextensive pairst) extending along a central cylindrical mast. The axisof symmetry of the central mast is preferably coincident with thecenterline of the borehole, and the helical antenna may extend along theborehole for several feet, say feet. In a preferred describedembodiment, each of the first and second pairs of helical conductingelements begin substantially at the midpoint of the central mast andwind in the same circumferential direction towards respective ends ofthe antenna. Situated within the central mast at the midpoint of theantenna adjacent to the ends of helical conductors is a microwavecoupler. Electromagnetic energy is coupled from a source (either locatedwithin the sonde or at the earths surface) through a single input/outputtransmission line connected to the microwave coupler and thence tocentrally located feed points for the antenna. In the aforementionedembodiment, each pair of helical elements is driven from the couplerusing energization feed points to produce in-phase propagation in theaxial direction from the coupler toward the ends of the antenna. As aresult,

the energy is radiated in planes transverse to the axis of the borehole(pancake beam, broadside radiation pattern) into the adjacent formation.

The present invention relates to improvements in the coupler for theaforementioned helical antenna system. Specifically, it is desirable, inmany applications, to correlate radar images obtained usingomnidirectional emitted radiation with radar images obtained from moredirectively emitted radiation. Further, radar images of differentlypolarized radiation, say vertical or horizontal, may also be a relevantfactor in resolving the texture of the flank of the salt dome.(Polarization relates to the direction of the electric field componentsof the radiated electromagnetic energy in its principal direction ofpropagation.)

It is an object of the present invention to provide a novel method andapparatus related to remote electrical and/or electromagnetic switchingof the energization feed points of the helical antenna by means ofactive control elements. It is desirable in the present invention thatthe radiation pattern of the antenna be easily changeable, say from anomnidirectional pattern in azimuth (pancake) to a more directive mode ofradiation, say a figure-eight pencil beam having twin, in-line directivelobes, in azimuth, even though the antenna is located thousands of feetbelow the earth's surface. It is also desirable that the directive lobesof the resulting figure-eight pencil beam of radiation be steerable inazimuth so as to focus to the direction from which radar images areobtained. It is further desirable that the direction of the polarizationof the wave be variable so that the radar images from common reflectionpoints provided in one polarizing mode (e.g., vertical) can be comparedwith radar images associated with another polarizing mode (e.g.,horizontal).

In accordance with the present invention, the radiation pattern of amultielemented helical antenna within a borehole penetrating the earthis systematically controlled, in azimuth, by operation of a switchingcoupler housed within the central mast of a helical antenna. In one modeof operation, the antenna system of the present invention is energizedat feed points near the midpoint of the central mast so as to emitradiation in a'broadside pattern (pancake beam) as first and secondpairs of electromagnetic waves travel from the coupler towardsrespective ends of the antenna. In this regard, each of the pairs ofelectromagnetic waves is controlled, in phase, so that traveling wavecurrent phasers of each pair of waves are in phase at the circumferenceof each pair of helical conductors as measured in any given azimuthaldirection. The emitted radiation, although omnidirectional in azimuth,is highly directional in planes normal to the axis of a well bore(elevational direction).

In a second mode of operation, the coupler is actuated through an activecontrol circuit. The associated circuitry can include rotor means whichwhen energized causes rotation of the coupler from one position toanother whereby the feed point distribution of the antenna isreoriented. More particularly, the feed points one of each pair of thehelical conductors are reversed. An electromagnetic wave guided by oneof a pair of helical conductors no longer propagates in the samedirection as the wave guided by the other of the pair, but travels inopposite axial directions. The diameter of the helix is chosen in such away that these two traveling waves are the same in phase at a certainangle in azimuth (direction of maximum radiation intensity) and areopposite in phase at another angle in azimuth (direction of minimumradiation intensity); thus the radiation from the antenna can be madedirectional. If the diameter of the antenna is chosen such that twodirections of maximum radiation intensity and two directions of minimumradiation intensity alternate, the resulting radiation pattern resemblesa bidirectional pencil beam in azimuth. The pattern, invention, remainshighly directional.

The present invention also provides for phase shifting means for theantenna of the present invention which can, likewise, be controlled byactive circuitry elements. As the phase shifting means is actuated (soas to shift the phase of the wave through various phase angles), theeffect is to steer" the figuregeight radiation pattern, in azimuth,along different directions relative to the axis of the well bore.Result: greater reliability as to the direction and distances to andfrom electromagnetic discontinuities within the salt dome relative to agiven azimuthal direction from the well bore. Radar images, in each modeof operation, can also be recorded and, later, cor related as todirection. In yet another embodiment of the present invention, the phaseshifters and 180") can be located within the helical antenna of thepresent invention. Specifically, if the two-stage phase shifters are setto operate in the 0 phase shift mode, the energy radiated from theantenna (whether operated in a broadside radiation mode or in thefigure-eight pencil beam mode) will be of horizontal polarization.Contrary, if the two-stage phase shifters are set to operate in the 180phase shift mode, the polarization of the energy will be vertical. Theresulting radar images in each polarization mode can be recorded and theresulting records can provide a new comparison quantity: the record withvertical polarization, as to common reflection areas, can be comparedwith a record with horizontal polarization, with the result that thetrue textural nature of the common reflection areas can be betterindicated.

Further objects and advantages of this invention will become moreapparent from the following detailed descriptions taken in conjunctionwith the accompanying drawings which form a part of this specification:

In the Drawings:

FIG. 1 is a partial section of an earth formation including a salt domepenetrated by a borehole and illustrates a logging sonde and otherapparatus for energizing the logging sonde and for transporting,depthwise, the sonde through the borehole penetrating the salt dome;

FIG. 2a is a representation of the face of a recording instrumentdisplaying the information to be derived from the logging sonde of FIG.1;

FIG. 2b is a two-dimensional plot of the near flank of the salt dome ofFIG. 1 as a function of depth;

FIG. 3 is an elevational view of the sonde of FIG. I, partially cutaway, to illustrate the improved antenna system of the presentinvention, the antenna adapted, in operation, to emit verticallypolarized radiation into the salt dome of FIG. 1;

FIG. 4 is a plot of traveling wave current phasers propagating, duringoperation of the antenna of FIG. 3, to provide for emission ofvertically polarized radiation;

FIG. 5 is a partial, perspective section of the salt dome of FIG. 1illustrating the radiation pattern provided by the helical antenna ofFIG. 3 operating in a first radiation mode;

FIG. 6 is an elevational view, partially cut away, of the helicalantenna of FIG. 3 particularly illustrating the switching coupler, inpartial schematic form, also illustrating active switching elements forchanging the polarizing pattern;

FIG. 6a is a detail of the switching coupler of FIG. 6 operating in asecond operating mode;

FIG. 6b is another detail of the switching coupler of FIG. 6 operatingin the second operating mode;

FIG. 7 is a sectional view taken along line 7-7 of FIG. 6 illustrating,in detail, operational principles of the helical antenna of the presentinvention when operating in the first operating mode;

FIG. 8 is a plot of traveling wave current phasers propagating, duringoperation of the antenna of FIG. 3, to provide for emission ofhorizontally polarized radiation;

FIG. 9 is a sectional view taken along line 9-9 of FIG. 6 il-Iustrating, in detail, the operational principles of the helical antennaof the present invention operating in a second operational mode toprovide a directive, in-line symmetrical radiation pattern (figureeight);

FIG. 10 is a partially perspective section of the salt dome of FIG. 1modified to illustrate the directive figure eight radiation patterngenerated by the helical antenna of the present invention operating inthe second operational mode;

FIG. 11 is an elevational view, partially cut away, of a modification ofthe helical antenna of FIG. 6, illustrating a switching coupler, inpartial schematic form, for changing the polarizing pattern from thefirst to the second operational mode;

FIG. Ila is a detail of the switching coupler of FIG. 11 operating inthe second operating mode; and

FIG. 12 is a sectional view of yet another embodiment of the helicalantenna of the present invention.

Referring now to FIG. I, a section of a salt dome 10 is shown penetratedby a borehole l1 offset from the center of the dome so as to be adjacentto one of its flanks. In order to accurately define the near sidewall ofthe dome 10 through controlled emission and reception of electromagneticenergy, an exploration sonde l3 incorporating an electromagnetic helicalantenna, generally indicated at 14, is transported along the borehole soas to be placed adjacent to different horizontal sections of the dome.The purpose of mapping the near side ofthe salt dome-sedimentaryinterface 12 is to identify those areas where oil deposits 15 are mostlikely to be found adjacent the sidewall of the dome.

To provide movement of the sonde 13 through the borehole 1 l, a loggingcable 16 is connected through sheaves 17 on derrick 18 to cable drum 19.Motor 20 powers drum 19 on hoist truck 21 to raise and lower the sonde.

As the borehole ll may be filled with a dense drilling fluid to preventintrusion of the earth formation into the borehole, the sonde 13 must befluidtight at the mating joints of the upper housing 22 with lowerhousing 23 supporting the helical antenna 14. The upper housing 22 isconnected to the lower housing 23 by union collar 24 as indicated.

Located within truck 21 is a console 25 downhole a power source andassociated coupling circuitry suitable for feeding timing signals alonglogging cable I6 to the sonde 13, as well as indicators for the sourceand coupling circuitry. Console 25 may also include surface recordingequipment including at least three indicators: one for impedance match,indicator 27; another for depth, indicator 28; and another for distance,indicator 29. The impedance match displayed on indicator 27 relates tothe power transfer between the helical antenna 14 and coupling circuitrywithin the sonde 13 as a function of their respective impedances, andthe matching is performed downhole during operation of the antenna.Depth indicator 28 shows the mapping depth of the antenna in theborehole 11 and is measured by pulley 30. The distance of the boreholeto the sidewall of the near side of the salt dome at each mapping depthis a function of the time between transmission and reception of theelectromagnetic energy by the antenna and is displayed on indicator 29.The information on the indicator 29 and the depth indicator 28 can besimultaneously recorded using a camera to produce a photographic plateof the type indicated at 31 in FIG. 2a. Plate 31 indicates the two-waytravel time (2T) for the emitted signals, and a series of thesephotographs may be reduced to a two-dimensional plot, such as shown inFIG. 2b, in which the location of the near wall of the salt bodyrelative to antenna axis 32 is represented by line 33 connecting mappedpoints A, B, C, D, E, etc.

Reference should now be had to FIG. 3. This figure depicts the interiorof sonde 13 in more detail illustrating how the helical antenna systemof the present invention utilizes timing signals fed from controlconsole 25 to initiate mapping operations. As indicated, an energizationsection 40, schematically illustrated, is positioned at the interior ofthe upper housing 22 and includes transmitter 41 which, in operation, isperiodically energized through modulator (pulser) 42 so as to generateelectromagnetic pulses of high power and relatively short duration.Since the helical antenna 14 downhole from the energization section 40is used for both transmitting and receiving, a switch arrangement 43,called a duplexer, is connected across the transmitter 41 and a receiver44 as indicated. As well understood, the duplexer 43 isolates thesensitive receiver 41 from the transmitter when energy is fed to theantenna and then connects the antenna to the receiver in the internalbetween pulses where the reflected energy is to be received.

VSWR coupler 45 is connected so as to sense the energy transmittedbetween the duplexer 43 and the antenna 14. The coupler 45 samples theenergy in the connecting transmission line to indicate the powertransferred to the antenna as a function of the relative impedances ofthe antenna 14 and the energization section 40.

The maximum power transfer from the transmitter to the antenna can beachieved in the present invention through a stub tuner 46 connected inparallel with VSWR coupler 45. Operation of tuner 46 is adjustablycontrolled by circuitry within the console 25 as the console operatormonitors the response of VSWR coupler 45 at indicator 27 (FIG. 1).

During operation of the antenna system in a first mode of operation, allhelices are driven from a midportion p-p of the antenna 14 by means ofaxially extending switching coupler 50 interior of central conductingcylinder (mast) 51 of the antenna 14. Switching coupler 50 includeselectrical conductors 47 and 48 for connection, uphole, to energizationsection 40. In the first mode of operation, the helical antenna 14 ofthe present invention can be thought of, fundamentally, as atransmission line directing first and second pairs of electromagneticwaves, each pair beginning at the midportion p-p of the antenna andhelically winding in opposite sense to the remote ends of the antenna.To provide for appropriate electromagnetic propagation, the helicalantenna 14 includes helical conductors 52, 53 and 56, 57 symmetricallydisposed on either side ofa reference plane pp, and central cylinder 51inwardly spaced from the helical conductors. Energy is fed from theenergization section 40 through the coupler 50 between the helicalconductors 52, 53 and 56, 57 and the central cylinder 51. Feed points tothe helical conductors, in the first operating mode, are indicated atfour feed points a, b, c and d adjacent to the midportion of theantenna.

In the second mode of operation, as explained in detail below, coupler50 is appropriately energized by the active control circuitry to providenew feed points for coupling energy from section 40 to the antenna 14 soas to provide a more directive radiation pattern. The characteristics ofthe resulting radiation patterns, in either the first or second mode ofoperation of the antenna, are dictated, of course, by thecharacteristics of the electric and magnetic fields of the energypropagating between the helical conductors 52, 53 and 56, 57 and thecentral cylinder 51. Accordingly, a brief description of the characterof the electromagnetic waves in each operating mode is in order.

First Operating Mode of the Antenna System During the first mode ofoperation of the helical antenna, the electromagnetic waves guided bythe first and second pairs of helices are driven at a, b, c and d andtenninated at both ends of the antenna. A first pair of electromagneticwaves can be thought of as propagating in helical paths betweencoextensive electrical conductors 52 and 53 and the central cylinder 51.Similarly, a second pair of waves canbe thought of as helicallypropagating in an opposite axial direction as between the remaining pairof coextensive helical conductors 56 and 57 and the central cylinder 51.Each propagating adjacent pair of electromagnetic waves have severalcommon characteristics. The phase of the electromagnetic wave guided byone ofa pair of helices is identical with that of the wave guided by theother of the pair at any azimuthal angle. Each wave thus contributes tothe intensity of the resulting radiated field. Further, the electricfield component radiated from the current on an incremental length ofawire is parallel to the direction of the incremental length of the wireand its magnitude is proportional to that of the current and its phaseis determined by the distance between the current element and the pointof observation of the field. As can be seen in FIG. 3, the helicalconductor lengths make an angle (0:), Le, pitch angle, with the planeperpendicular to the axis of the cylinder 51. To provide a constantpitch angle a, the helical conductors are radiallyspaced from thecentral conductor-51 a constant radial distance (8,). The resultantfield can be calculated by a vec-' torial summation of the contributionsof all current elements. Accordingly, the pairs of traveling wavecurrents propagating in opposed axial directions on the antenna may beadapted to interact as a function of their position along the antenna,as measured in opposed but equal axial distances from the central planep-p so as to provide a particular type of polarization.

FIG. 4 illustrates, in partial schematic form, traveling wave currentphasers propagating on azimuthally aligned portions of each pair ofelectrical conductors 52,53 and 56,57 of FIG. 3. To provide forvertically polarized radiation, the traveling wave phasers along thesame azimuthally aligned segments of one set of conductors (sayconductors 52 and 53) must be of the same magnitude and of the samepolarity relative to one another and, most importantly, must be ofopposite polarity to symmetrically located traveling wave currentphasers propagating in an opposite direction on the oppositely extendingpairs of helical conductors. Correlation of the resulting currentphasers, for this example, is taken at equal distances from the centralaxis 32 of the antenna measured in the same mean azimuthal direction. Asshown, the current phasers are represented by arrows 58, 59, 60 and 61.These arrows indicate that the instantaneous current phasers found inaxially spaced sections of each pair of helical conductors can bedefined by an imaginary geometrical figure, the figure extending alongthe antenna and having a wedge-shaped cross section, one comer of whichis located on the antenna axis 32 and the two remaining corners of whichare located on parallel lines 34 and 35. (Lines 34 and 35 are radiallyspaced from but parallel to the axis 32.) The current vectors may beresolved into vertical vectors 58b, 59b, 60b and 61b and the horizontalaligned vectors 58a, 59a, 60a and 61a. Accordingly, the first pair ofwaves, say propagating between helical conductors 52 and 53 and thecentral cylinder 51, interact with the second pair of waves (on theoppositely aligned segment of the antenna) to generate verticallypolarized electromagnetic energy.

The polarization of the resulting radiated field relates to thedirection of the electric field components of the radiated field in theprincipal direction of propagation. FIG. 5 illustrates the geometry ofthe resulting electromagnetic radiation provided by the antenna 14 ofFIG. 3, the vertical polarization character of which having beenpreviously described in some detail in FIG. 4. As shown in FIG. 5, theradiated field (from a broadside radiating helical antenna locatedwithin sonde 13) is in the form of a geometrical solid revolution. Asmall section of energy (called a wave front) is generally indicated at26 perpendicular to the direction of travel of the energy. The electricfield components of the energy are seen to be vertically polarized.

FIG. 6 is an elevational view, partially cut away, of the helicalantenna of FIGS. 3 and 5 illustrating, in schematic form, the elementsof switching coupler 50.

It is contemplated that the switching coupler 50 of the presentinvention will be constructed of laminated, flat metallic plates fittedtogether along their adjacent broad sides (flats) to form a housing. Thehousing is illustrated as being cylindrical and having a sidewall 49a insurface contact with the interior wall of central cylinder 51. End wall49b of the housing is fitted with connectors 65 and 66 ultimatelyconnecting, uphole, through conductors 47 and 48, respectively, to theenergization section 40. As conductor 48 is a conventional coaxial line,it is convenient to couple the outer conductor thereof to the centralcylinder 51 of the antenna 14, the connection point occurring,preferably at connector 66. In that way, the conducting path is throughconnector 66 and the coupler housing to the cylinder 51. Cavities (notshown) in adjacent broad walls of the metallic plates (each cavity beingpreferably semicircular in cross section) are provided by milling ormachining operations prior to their assembly to form the couplerhousing. These cavities serve as cntryways for receiving (andsupporting) the inner conductor of the input/output conductor 48 bywhich electromagnetic energy is transferred between the helical antennaand the energization section 40. The inner conductor of conductor 48 canbe sup ported within the cavities by annular insulating discs. In asimilar manner, at the exterior of the antenna, the helical conductorscan be insulated from the central cylinder 51 by radially extendinginsulating posts 64. For an example of laminated coupler constructiontechniques, see the aforementioned patent of O. A. Fredriksson et al.,US. Pat. No. 3,449,657. After passage through the coupler 50, the energycan be coupled, in turn, between the helical conductors 52, 53 and 56,57 as at the feed points a, b, c, and d, and the central cylinder 51 sothat traveling wave current can be established along the antenna in themanner previously mentioned. All radio wave cables inside the coupler 50function as coaxial cables unless otherwise specified.

Mechanical arrangement of the inner conductor of the conductor 48interior of coupler 50 forms an important aspect of the presentinvention. As shown in FIG. 6, the inner conductor of conductor 48comprises a series of split conductors split into a series of extensionsfor multiple coupling of energy to the antenna. In particular, linesections 68 and 69 form the first split segments of the series of splitconductors and are seen, at point F, to connect in parallel to the maininput segment 67. Each section 68 and 69, in turn, is split intoextensions to form the correct mode of excitation of the helicalantenna. In more detail, line section 68 is split into the two U- shapedextensions 70 and 71. Extension 70 extends longitudinally from itsattachment point G to line section 82 at an arcuate area near the upholeend of the coupler and then loops back toward the midportion p-p of thecoupler for connection to helical conductor 56. Extension 71 extendsfrom line section 68 at point G in an opposite direction to that ofextension 70 to an arcuate area near the opposite downhole end of thecoupler and then loops back toward the midportion p-p for connection tohelical conductor 52. The remaining helical conductors 53 and 57 connectto line section 69 through diagonally extending extensions 72 and 73.The feed points a, b, r: and d, viewed in elevation (FIG. 6) are seen todefine an imaginary parallelogram lying in a common plane and havingparallel opposite sides centered at the midplane p-p. Diagonal couplingpoints b and d connect to extensions 72 and 73 and form one set ofcoupling points. The other two diagonal coupling points a and connect tothe U-shaped extensions 71 and 70, respectively, and form a second setof coupling points.

Energization of the antenna with energy of correct polarity depends uponthe relative line length of the differing combinations of splitconductors forming extensions from input/output conductor 48 within thecoupler 50. In more detail, if the combined length of section 69 andextension 72 or 73 is exceeded by the length of section 68 incombination with extension 70 or 71 by a distance, say equal to MAf/Zwhere M is any cardinal number and M is the operating wavelength ofenergy in the salt dome, correct feeding conditions exist. When M is anodd number the direction of the polarization is vertical, and when M isan even number the direction of the polarization is horizontal. Eachpair of helical conductors 52, 53 and 56, 57 have one conductor of eachpair fed 180 out of phase with the other helical conductor comprisingeach coextensive pair. ln this regard, a variable phase shifter 75 maybe located, in series, in section 68 to aid in the control of phase ofthe energy at the feed points a, b, c and d. The operation of phaseshifter 75 is controlled by signals generated uphole from the antenna,as within console operating in association with coupler controller 39positioned within energization section 40 (FIG. 3). The control signalsenter coupler 50 via conductor 47 at end wall 49b of the couplerhousing, and are conveyed to the phase shifter interior of the couplerby wire conductor 760. It should be noted that while along successivetransverse sections of the helical conductors the polarity of thetraveling wave current phasers undergoes periodic change, the relativepolarity along common azimuthal directions remains in phase to generatethe desired vertically polarized energy output. As shown in HO. 7, whichis a section taken along a transverse plane through the antenna of FIG.6, the current phasers l and l along separate helical conductors 56 and57- are depicted in partial schematic form. In the same transverseplane, I, and are of opposite polarity, but along equalized conductorincrements in common azimuthal directions measured from axis 32 of theantenna, they are in phase. Vector summation of the phasers l and lproduces the results depicted at the periphery of the helicalconductors, which indicate for the azimuthal angle 6 equals O360 l, andI: are in phase.

However, the polarization mode of the helical antenna of FIG. 3 is notlimited, in operation, to vertical orientation only but can be changedto a horizontal configuration by operation of active elements withinswitching coupler 50. -ln many borehole applications, signal resolutioncan be increased by changing the polarizing mode of the radiated energy,say from a vertical to a horizontal orientation, while the sonde remainspositioned many feet below the earths surface.

Referring again to FIG. 6, extension 73 of coupler 50 is seen to beconnected to helical conductor 57; located at an intermediate positionalong extension 73 in series therewith, is a two-stage phase shifter 77electrically controlled by the control circuitry through wire conductor76b and connector 65. For the helical conductor 52 a second phaseshifter 79 is located, in series, along extension 71 terminating incontact with the helical conductor 52. Phase shifter 79 is controlled byuphole control circuitry through wire line 76c. Both phase shifters 77and 79 have two settings: 0 phase shift setting and 180 phase shiftsetting. At 0 phase shift position the phase of the energy being coupledto the respective in series helical conductors 57 and 52 is notaffected. However, when the phase shifters 77 and 79 are changed to 180phase shift setting by means of timing signals from uphole of theantenna, each phase shifter causes 180 phase shift of the energy beingcoupled to the helical conductor. As a result, the phases of the currentat c and b are in phase and those ofa and d are also in phase, butphases at c and d are 180 out of phase. With these phase arrangements ofthe direction of the polarization becomes horizontal. FIG. 8 shows thevector diagram of the current on the helical wires. As shown in FIG. 8,the traveling wave current phasers on each pair of helical conductors52, 53 and 56, 57 along the same azimuthally aligned segments must be ofequal magnitude and, most importantly, must be of the same polarity asrespective traveling wave current phasers on oppositely extending pairsof helical conductors at equal distance from the midpoint plane p-p andin a common azimuthal direction relative to axis 32. When theseconditions exist, traveling wave phasers represented by the arrows 85,86, 87 and 88 are generated along the antenna and these phasers areresolvable into reinforcing horizontal vectors a, 86a, and 87a and 88aand into cancelling vertical vectors 85b, 86b, 87b and 88b. The resultis the generation of horizontally polarized radiation. Thecircumferential turn length of the helical conductors should also be aconstant value equal to a value related to the phase of the adjacentpairs of electromagnetic energy. In that way the pairs ofelectromagnetic waves can be correctly phased for interaction in boththe longitudinal and radial directions along the length of the antenna.A common turn length of NA, is preferred where N is any integer,preferably 1, and A, is the operating wavelength of the electromagneticenergy in the adjacent earth formation such as a salt dome.

During operations it is desired that reflections from the ends of theantenna be minimum. For this purpose, matched loads 90, 91, 92 and 93within the coupler S0 of FIG. 6 are located adjacent to the ends of thehelical conductors 52, 53 and 56, 57.

Second Operating Mode of the Antenna System During the second mode ofoperation of the helical antenna of the present invention the first andsecond pairs of electromagnetic waves can be thought of as propagatingalong the antenna in a helical fashion, in the manner previouslydescribed, but they interact in a much more complex manner. It isconvenient to think first of the direction of the propagation of eachwave comprising each pair of waves: In the second operating mode of theantenna, one of the waves comprising each pair of waves propagates in anaxial direction opposite to that of the other wave. First, consider thewaves which propagate in the same direction. The wave associated withhelical conductor 53 begins at the feed point b in both operating modesand propagates between the conductor 53 and the central cylinder 51 inthe manner previously described. Similarly, the wave associated withhelical conductor 57 begins at feed point d and propagates in the mannerpreviously described. Next, consider the waves whose directions ofpropagation have been reversed. The wave associated with helicalconductor 52, in the second mode, propagates from a new feed point Eadjacent to the end wall 490 of the coupler housing toward the midplanep-p of the antenna. Similarly, the wave associated with helicalconductor 56 propagates from a new feed point E adjacent to end wall 49bof the coupler toward the midplane p-p.

To provide reversal of wave propagation described above, the switchingcoupler 50 of FIG. 6 is provided with active control elements, such asdual rotary switches 80 and 81. On command, these switches can changethe feed points associated with the helical conductors 52 and 56in thefollowing manner. The rotary switches 80 and 81 are seen in FIG. 6 inseries with U-shaped coaxial extensions 71 and 70, respectively at theapex of the arcuate loops of these extensions. Each rotary switchconsists of two pairs of curved conductor segments 82 and 83 positionedwithin a housing 84. Housing 84 (and, more particularly, curved segments82 and 83) can be repositioned to provide the first and second operatingmodes for the helical antenna:

(I) In the manner depicted in FIG. 6 (first operating mode of theantenna system) the curved segments 82 of the switches 80 and 81 areconnected in series with stationary gate points x, w and m, n at theapex of the U-shaped coaxial extensions 71 and 70, respectively, toprovide coupling of the energy to helical conductors 52 and 56 at thefeed points a and c adjacent to the midplane p-p of the antenna. Curvedsegments 83 of the rotary switches 80 and 81 are connected in serieswith stationary gate points y, z and r, s so as to terminate the helicalconductors 52 and 56 with the matched loads 90 and 92 and the remoteends (with respect to plane p-p) of the helical conductors 52 and 56.Because of the interconnection of the loads 90 and 92, after the firstand second pairs of electromagnetic waves have propagated from the feedpoints a, b, c and d adjacent to midplane p-p to the remote ends of theantenna, reflection of the energy back toward the midpoint of theantenna is minimized.

(2) In the second operating mode of the antenna, the rotary switches 80and 81 are actuated by control signals originating uphole from theantenna and entering coupler 50 via connector 65 and being conveyed tothe switches 80 and 81 by line conductors 76d and 762, respectively. Theeffect of the actuation of rotary switches 80 and 81 is to reorient thecurved segments 82 and 83 of each rotary switch 80 or 81 to newpositions depicted in FIGS. 6a and 6b. In detail FIG. 6a depicts rotaryswitch 81 in an angular position required for the second mode ofoperation of the helical antenna. In still more detail, the curvedsegment 82 of the switch 81 is seen to be placed in a new locationcircumferentially spaced about 90 from the stationary gate points In andn of FIG. 6. In that way, peripheral stationary gate points m and r, inFIG. 6a, are now electrically connected via segment 82. Thus, as seen inphantom line in FIG. 6, energy can be coupled to helical conductor 56 atfeed point E located adjacent to end wall 49!: of the coupler housing.The resulting electromagnetic wave associated with the helical conductor56 will propagate from the feed point E toward the midplane p-p;adjacent to the midplane p-p the wave will then be coupled via a portionof extension 70 through segment 83 of the switch 81 (between gate pointsIt and s) to matched load 92. In FIG. 6b, curved segment 82 of theswitch 80 has likewise been placed in a new location from that depictedin FIG. 6. In more detail, peripherally spaced stationary gate points xand y are now electrically connected by curved segment 82. In that way,energy passing into the coupler 50 of FIG. 6 is conveyed along a segmentof extension 71 to the switch and thence along segment 82 (shown inphantom line in FIG. 6 between stationary gate points x and y) to theremote end of helical conductor 52, i.e., to feed point E marking thefeed point of helical conductor 52 in the second mode of operation.Thus, the resulting electromagnetic wave associated with helicalconductor 52 will propagate from the feed point B at the remote end ofthe antenna towards the midplane p-p, the wave then being dissipated atmatched load by passage along a portion of segment 71, and segment 83 ofswitch 80 (between stationary gate points w and z) to matched load 90.Thus, the effect of operating the rotary switches 80 and 81, in tandem,is to switch the feed points of the helical conductors 52 and 56 fromfeed points a and 0 adjacent to midplane p-p to the feed points E and Eat the ends of the antenna. The resulting pairs of helical conductorscan interact in a manner to create a far more directive radiationpattern in the manner described in more detail hereinafter.

It is also evident, irrespective of the mode of excitation of thehelical antenna via rotary switches 80 and 81, that the matched loads90, 91, 92 and 93 are always located at the terminating ends of thehelical conductors 52,53 and 56,57. Accordingly, irrespective of thedirection of travel of the electromagnetic waves comprising each of thefirst and second pairs of electromagnetic waves, reflections of suchwaves from the remote terminals of the antenna are minimized.

In the second mode, each pair of waves, though complex in operation,must propagate so that their phases, at varying azimuthal angles,combine in a predetermined manner although the waves themselves varyperiodically with distance from the midplane p-p of coupler 50. Aspreviously described, the electric field component of each pair of wavescan be thought of as lying parallel to an incremental length of helicalconductor from which the energy radiates. Thus, each electric fieldcomponent of the propagating wave can be thought of as having aninstantaneous traveling wave current phaser in the incremental length ofhelical conductor. Referring now to FIG. 9, the traveling wave phasersI, and I at varying azimuthal angles 9 along helical conductors 56 and57 measured with respect to axis 32 of the antenna are illustratedduring the second mode of operation of the helical antenna of thepresent invention. As indicated at azimuthal angles (6) 0 and thephasers are algebraically additive, i.e., at 0 equals 0 and 180, thephasers l and I, are in phase; but at the azimuthal angles (6) of 90 and270, they are 180 out of phase and therefore cancel.

FIG. 10 is a perspective section of the salt dome 10 of FIG. 1illustrating the directive radiation pattern, in azimuth, generated bythe helical antenna of the present invention (operating in the secondoperating mode) housed within sonde 13. As shown, the radiated field, inelevation, remains highly directional in a plane perpendicular to theaxis 32 of the antenna. The section of energy (called a wavefront) isgenerally indicated at 36 and defines diametrically located inline beamlobes 37a and 37b, which define a figure-eight radiation pattern. Asshown, the lobes 37a and 37b are diametrically located with respect tothe axis 32 of the antenna and have a quasi-elliptical shape, one end ofwhich lies on the axis 32 of the antenna, and a rather narrow effectivebeam width, say less than 10. (The antenna beam width may be arbitrarilydefined as the angular distance between two directions to space wherethe power has half the maximum value.) The electric field componentsofthe energy are seen to be vertically polarized.

The figure-eight radiation pattern of FIG. 10 need not be fixed inazimuth. By operation of phase shifting means 75 of FIG. 6, in themanner explained below, the lobes 37a and 37b of the radiation patterncan be steered (rotated) through various azimuthal angles relative tothe axis 32 of the antenna.

Referring again to FIG. 6, it will be recalled that phase shifting means75, previously described with the antenna of FIG. 6,

is useful in properly phasing energy coupled from or to the antenna soas to provide correct phase relationships of the travelingelectromagnetic waves propagating along the antenna. After correctphasing has occurred, the phase shifting means 75 can also besystematically changed from a setpoint value so as to rotate the primarydirection of the antenna lobes 37a and 3715 through various azimuthaldirections relative to the borehole. Thus, after the omnidirectionalfirst operating mode is used for searching for the reflectingdiscontinuities and reflecting discontinuities are found, the operatingmode of the antenna can be easily switched to its more directive secondoperating mode to acquire the detailed dimensions of thediscontinuities.

Polarization of the figure-eight radiated field into the surroundingearth formation also can be easily changed utilizing the two-stage phaseshifters 77 and 79 of FIG. 6. As previously described, phase shifter 77electrically connects uphole by wire conductor 76b to control circuitrywithin energization section 40; likewise, the phase shifter 79electrically connects through wire conductor 76c to the same controlcircuitry. After the variable phase shifting means 75 has beenestablished at a setpoint level, the phase shifters 77 and 79 can beoperated in their second setting, 180 phase shift setting. Of course, intheir phase shift setting the phase shifters 77 and 79 couple energydirectly to the helical conductors 52 and 57 without change in the phaseof the coupled energy. However, in the second operating mode these phaseshifters are activated through control signals in the circuitry upholefrom the antenna, each phase shifter 77 and 79 operates to shift thephase of the energy 180 over that of the first 0 phase shift setting.The result: Traveling wave current phasers on similarly aligned portionsof each pair of helical conductors are reoriented to provide horizontalpolarized radiation. That is to say, when the phase shifters 77 and 79are at 0 phase shift setting, the figure-eight radiation pattern isvertically polarized, as depicted in FIG. but when they are at 180 phaseshift setting, the direction of the polarization is converted tohorizontal. The resulting radar images can be recorded at the console 25at the earths surface. If these resulting records are compared as afunction of common azimuthal directions, it has been found that thetextural nature of the discontinuity can be inferred. Critical factorsof comparability include comparative signal amplitude, total absence ofcomparative signals, differing arrival times, etc. For example, for aparticular discontinuity within the formation, say at the boundary ofthe salt dome, the energy in the vertical mode may be reflected andreceived as a single reflection signal, while for the samediscontinuity, horizontally polarized energy may not provide areflection signal at all.

FIG. 11 illustrates a modification of the coupler 50 of FIG. 6. In FIG.11, the modified coupler 50' comprises a series of split segments of theinner conductor of the coaxial conductor 48'. The initial or maininput/output segment of the split segments being designated by thenumber 67 so as to simplify the construction of the coupler 50.,Maininput/output segment 67 enters the coupler 50' at coni'iector 66 andinterior of the coupler, splits at point k into three separate segments101, 102 and 103. Segment 101 connects point It to helical conductor 53at feed point b. Segment 101 connects point It to helical conductor 53'at feed point b. Segment 102 connects point k to helical conductor 57 atfeed point d. Segment 103 extends from point k at an acute angle withrespect to segments 101 and 102 and, after a rather tortuous path,ultimately connects to helical conductors 52 and 56 at the feed points aand 0, respectively. Between point k and feed points a and c, thesegment 103 can be seen to resemble the mirror image of the letter Rlying on its side. A single rotary switch 95 is located at the arcuateportion 1030 of the segment I03. Beyond the rotary switch 95, the baseleg 10312 of the segment I03 terminates at feed points a and c of thehelical conductors 52 and 56'. In operation, rotary switch 95 iscontrolled by control signals originating at control circuitry and therotary switch is electrically controlled by the aforementioned circuitryby wire conductor 96a which extends into coupler 50' at connector 65. Inthis embodiment one rotary switch 95 duplicates the functions of thedual rotary switches and 81 of FIG. 6, but has similar constructionalfeatures: As shown in FIG. 11, the rotary switch includes first andsecond arcuate wire segments 97 and 98 housed within metallic housing99.

To provide the proper mode of excitation to the helical antenna thecoupler 50 operates in the following manner:

(I In the first operating mode the rotary switch 95 is positioned asshown in FIG. 11 so that wire segment 97 couples stationary gate pointsm and n of the arcuate portion 1030 of the segment 103 in electricalcontact with the main input/output segment 67'. In that manner energycan be coupled to the helical conductors 52', 53', 56' and 57' at thefeed points a, b, c and d so as to provide for the propagation of firstand second pairs of electromagnetic waves from the midplane p'-p inopposite axial direction towards the ends 890 and 89b of the coupler50'. As the waves associated with the helical conductors 52' and 56reach the ends of these conductors (adjacent to the ends 89a and 89b ofthe coupler), they are seen to be dissipated interior of the coupler byconnecting the helical conductors 52' and 56' to matched load 111.Matched load 111 connects to the ends of the helical conductors 52' and56' by means of a rather long common axially extending wire segment 107which connects at about its midpoint through trunk segment 108 to thematched load 111. In that way segment 98 of the switch 95 can be seen toconnect the stationary gate points r and s of the trunk segment 108together. In similar manner, matched loads 112 and 113 connect to theremote ends of the helical conductors 53' and 57', respectively,interior of the coupler 50'. During propagation in the first operatingmode, energy will be radiated from the antenna in the omnidirectional(pancake) radiation pattern; similarly, energy will be received fromdiscontinuities within the adjacent formation in the manner previouslydescribed.

(2) In a second operating mode the rotary switch 95 is positioned asshown in FIG. 11a (and in phantom line in FIG. 11) whereby arcuatesegment 97 connects stationary gate points r and m together so that theinput section of the arcuate segment 103a (i.e., the section closest tothe main input/output segment 67') is coupled, electrically, to axiallyextending segment 107 through trunk segment 108. It will be recalledfrom a discussion of FIG. 11 that segment 107 extends nearly the totallength of the coupler S0 and terminates adjacent to the ends 890 and 89bin contact with the helical conductors 52 and 56'. It can now beobserved that points 0 and 1 represent the initial feed points of theenergy to the helical conductors 52' and 56' in the second operatingmode. In the second operating mode, the remaining arcuate segment 98 isindicated as connecting the stationary gate points n and n together.Since the base leg 10312 of the segment 103 (FIG. 11) is now connecteddirectly to matched load 111, electromagnetic Waves associated withhelical conductors 52' and 56 will be properly terminated so as toprevent the production of reflected waves from the end points of thehelical conductors.

Thus, the effect of actuating rotary switch 95 to assume the positionillustrated in FIGS. 11 and 11a is to switch the feed points of thehelical conductors 52 and 56' from the feed points a and c to theaxially remote feed points 0 and (FIG. 11). In this manner, oppositepropagation directions of the electromagnetic waves constituting thefirst and second pairs of waves can be initiated so as to provide eitheran omnidirectional radiation pattern (first operating mode) or a moredirective figure-eight radiation pattern (second operating mode).

It is evident, irrespective of the mode of excitation provided viaswitch 95, that the matched loads I11, 112 and 113 are always located soas to correctly terminate the helical conductors 52, 53' and 56, 57'. Inthat way reflections of such waves (from the end points of the helicalconductors) are minimized. It is also evident that operation of variablephase shifter 115 located in series with leg 103c of segment 103, aswell as the operation of two-stage phase shifters 116 and 117 or 180)located in segment 102 and segment 107, respectively, can operate toachieve the radiation patterns of the type previously described. Notonly can the variable phase shifter 115 be useful in matching the phaseof the energy being fed to the antenna in the first operating mode, butit can be useful in the second operating mode to steer the figure-eightradiation pattern through various azimuthal directions with respect tothe axis of the antenna Similarly, the phase shifters 116 and 117 withtwo settings (0 or 180) are useful in providing energy in either thevertical or horizontal polarizing mode in the manner previouslydescribed.

Modifications It should be mentioned in my copending application, Ser.No. 876,479 (Passively Controlled Duplexer Coupler Applied to A HelicalAntenna For Use In A Borehole Penetrating An Earth Fonnation, filedconcurrently with the instant application) there is disclosed a helicalantenna system in which the functions of the duplexer and direct-linkagecoupler have been combined into a single, compact unit. Briefly, in theaforementioned copending application, first and second pairs of helicalelements of the antenna receive energy from a source located uphole fromthe antenna through a novel duplexer-coupler located within the interiorof the central mast of the helical antenna system. The duplexer-coupleremploys no active elements in performing its dual functions: (1)coupling high power energy from the transmitter to the antenna whileisolating such energy from the receiving circuitry and (ii) couplingreturn echoes of transmitted energy to the receiver while isolating suchsignals from the transmitter. In the aforementioned application, I haveprovided passive, power dependent switches within the coupler which arepositioned in appropriate split arms of the coupler and are onlyactivated as a function of the power level of the energy passing throughthe switches No active elements are used. Simultaneously, by constructedthe distances of varying coaxial arms within the coupler, the energyinput to the helical antenna can be properly phased so as to provideradiated energy polarized in a single direction, say vertically withrespect to the earth's surface, yet, in the receiving mode, the couplercan reactivate one helical conductor of each conducting pair so that theantenna has a maximum receiving sensitivity when the incident reflectionsignal is of the other polarized mode, say horizontal. In that way theantenna system can provide cross-polarized reflection images. Byrecording these images and comparing them, increased resolution as tothe true nature of the electromagnetic discontinuity can be obtained.

Concepts described and claimed in the aforementioned application can becombined with the invention described and claimed in the instantapplication, without departing from the intent or scope of the presentinvention. In that way, the advantages of using a singleduplexer-coupler for energizing a helical antenna system can be combinedwith the advantages of using active control elements within theduplexer-coupler for switching the radiation pattemin elevation from,say, an omnidirectional pattern to a more directional figure-eightradiation pattern, even though the antenna system is located thousandsof feet below the earth's surface.

Further, while certain preferred embodiments of this invention have beenspecifically disclosed, it should be understood that still othervariations will be readily apparent to those skilled in the art. As afurther example, instead of using a helical antenna having coextensivefirst and second pairs of helical conductors winding in oppositedirections about a central mast, it is evident that a single pair ofhelical conductors 118 and 119 as shown in FIG. 12 could be utilized incombination with a central mast 120 to form a radiation receivingantenna system. As shown in FIG. 12, helical conductors 118 and 119 aswell as central mast 120 are electrically coupled to and actuated byuphole systems connected to the antenna by means of coupler 123. Coupler123 is positioned within central mast and consists of a central housing124 through which input/output segment 127 of the inner conductor ofcoaxial line 125 is split into a series of subsegments to providedesired modes of excitation to the antenna as described hereinafter. Itis evident that the outer conductor of the coaxial line 125 is connectedthrough the housing 124 to the mast 120, at the connector 126.Input/output segment 127 is seen to be split at point k to formsubsegments 128 and 129. Subsegment 129 extends longitudinally along theaxis of the antenna for connection to the helical conductor 118.Subsegment 128 is seen to extend through rotary switch 131 forconnection to helical conductor 119. Rotary switch 131 can be energizedto have the following operating conditions:

l In a first operating mode (generation of omnidirectional radiation)switch 131 provides direct linkage between the helical conductor 119 andthe input/output segment 127. As seen, the curved segment 132 of theswitch 131 connects the former with the latter. In that way, a pair ofelectromagnetic waves can be seen to emanate at feed points q and uadjacent to the end wall 134 of the coupler. These waves, as theypropagate in tandem adjacent to the helical conductors 118 and 119,provide the irradiation of energy in the omnidirectional mode-that is,in planes perpendicular to the central axis of the antenna. The otherarcuate segment 133 of the switch 131 is seen to connect the remote endof helical conductor 119 to matched load 135. Another matched load 136is located at the remote end of helical conductor 118.

(2) In the second operating mode (generation of figureeight radiation)the rotary switch 131 connects, by arcuate segment 132 shown in phantomline, the stationary gate points ill and B together so that the incomingor outgoing energy relative to the switch 131 can be coupled from theinput/output segment 127 to the opposite end of the helical conductor119. In that way energy can be fed to the helical conductor 119 at feedpoint i adjacent to end wall 138. In the second operating mode, a pairof electromagnetic waves propagate in opposite directions relative toeach other. Result: traveling wave current phasers interact in themanner previously described to produce in planes perpendicular to thelongitudinal axis of the antenna a figure-eight directive radiationpattern. The arcuate segment 133 of the switch 131 (as shown in phantomline) connects the terminal end of the helical conductor 119 to thematched load 135.

It is also desirable that a variable phase shifter 139 be positioned inseries with the subsegment 129. If the variable phase shifter 139 is sopositioned, the relative phase of the pair of electromagnetic waves canbe varied to insure proper interaction whether the antenna is operatingin the first or second operating modes. It is evident in the firstoperating mode that the phase of the energy at feed points q and u mustbe out of phase to insure proper interaction of the traveling wavecurrent phasers. In the second operating mode, the phase of the energycan also be varied by phase shifter 139 to steer the figure-eightdirectional beam in azimuth.

Iclaim:

1. In a subsurface earth formation exploration tool for logging an earthformation penetrated by a borehole to approximate the distance anddirection to an electromagnetic discontinuity in said formation fromsaid borehole by measuring the two-way travel time of electromagneticenergy generated by means of an energization circuit including a sourceof electromagnetic energy and a control circuit means, uphole orinterior of said tool, a helical antenna supported within said tool forirradiating said formation with a two-condition, selectively variablepattern of electromagnetic radiation, in azimuth, comprising:

a. a central conducting element axially elongated substantially alongthe axis of said well bore,

b. at least N pair(s) of helical conducting elements wound axially alongand radially spaced outwardly from said central element, where N is anywhole number greater than zero, each helical conducting element of eachN pair(s) of elements winding in the same circumferential directionabout said central element toward a common end of said central element,

c. switching coupler means positioned interior of said central elementand including a plurality of feed points positioned along said helicalantenna for coupling, from said energization circuit, electromagneticenergy between said central element and said N pair(s) of helicalelements, and condition means operatively connected to and controlled bysaid control circuit means for coupling, among said plurality of feedpoints, electromagnetic energy to a first subset of feed points in afirst discrete operating state so as to produce, along said antenna, atleast N coextensively propagating pair(s) of electromagnetic wavespropagating in phase along said antenna in a common axial direction,said N pair(s) of electromagnetic energy radiating from said antenna inphase into said adjacent earth formation in planes substantiallytransverse to the axis of said well bore to produce a uniform,omnidirectional transverse radiation pattern,

said conditioning means, in a second discrete operating state, inresponse to a change in condition within said control circuit means,sequentially decoupling N adjacent feed point(s) of said subset of feedpoints from operative contact with said energization circuit and thencoupling N end feed point(s) to said energization circuit whereby Npair(s) of coextensively propagating electromagnetic waves are produced,said N pair(s) of electromagnetic waves, in said second discreteoperating state, each traveling in opposite axial directions along saidantenna but interacting so as to irradiate, into such adjacent earthformation, a highly directional, figure-eight pattern of electromagneticenergy.

2. The helical antenna of claim I in which N 1, whereby one pair ofelectromagnetic waves is generated Fetween one pair of helicalconducting elements and said central element, and, in said firstdiscrete operating state, propagate along said central element in acommon axial direction but, in said second discrete operating state,said waves of said one pair of waves propagate in opposite axialdirections along said central element.

3. The helical antenna of claim 1 in which N=2, whereby first and secondpairs of electromagnetic waves are generated between first and secondpairs of helical conducting elements and said central element,

in said first discrete operating state, each pair of electromagneticwaves begin substantially at the midplane of said central element andcoextensively propagate in the same axial direction toward a common endof said central element to generate said omnidirectional transverseradiation pattern in said formation, said first pair propagating in anopposite axial direction as said second pair ofwaves,

in said second discrete state, an individual first or secondelectromagnetic wave of each first and second pairs of waves propagatesin coextensive relationship with another individual wave of the samepair of waves but in an opposite axial direction as said another wave soas to generate said directional, figure-eight pattern of electromagneticradiation.

4. The helical antenna of claim 3 with the addition of phase shiftingmeans electrically connected to said control circuit means and operatedthereby, in said second discrete operating state, to selectively shiftthe phase of said first and second pairs of electromagnetic wavespropagating between said first and second pairs of helical conductingelements and said central element so as to cause selective steerage, inazimuth, of said figure-eight pattern of radiation relative to the axisof said borehole and thereby aid in identifying the azimuthal directionof said discontinuity relative to said borehole axis.

' 5. The helical antenna of claim 4 with the addition of first andsecond phase shifting means equipped with two settings of and 180 phaseshifts electrically controlled by'said control circuit and operative insaid first and second discrete operating settings to shift thepolarizing mode of said first and second pairs of electromagnetic wavesfrom a first to a second polarizing mode.

6. The helical antenna of claim 3 in which said switching coupler isfurther characterized by a series of segmented conductor means interiorof said central element, said series of conducting means joined togetherat a series of junctions interior of said central element andterminating in a plurality of terminating segments,

said first subset of feed points in said first discrete operating statecomprising four centrally disposed feed points connecting, through saidterminal segments, said first and second pairs of helical conductingelements and said central element to said source of electromagneticenergy, said four central feed points defining corners of an imaginaryparallelogram lying in a plane through said switching coupler, one setof adjacent diagonally located feed points being connected in serieswith said source of electromagnetic energy through selected lengths ofsaid segmented conductor means so as to couple electromagnetic energy tosaid helical elements in a phase opposite to that coupled to theremaining set of diagonally located feed points.

7. The helical antenna of claim 6 in which said condition means isfurther characterized by first and second switch means each locatedadjacent to a remote end of said central element as measured from themidplane thereof, in said second discrete operating state, said firstand second switch means operating to decouple said one set of adjacentdiagonally located feed points from contact with said source of energyand then couple said source to said end feed points remote from saidmidplane to launch electromagnetic waves along said antenna in anopposite axial direction than in said first discrete operating state.

8. The helical antenna of claim 7 in which first and second switchmeans, in changing from said first to said second discrete operatingstates, simultaneously reorient matched loads located at remote ends ofsaid central element to contact the terminal ends of said first andsecond pairs of helical conducting elements so as to prevent reflectionsof terminating electromagnetic waves back through said antenna towardsaid feed points.

9. The helical antenna of claim 6 in which said plurality of terminalsegments of said conductor means interior of said central elementconnect, in series, to said source of electromagnetic energy through avariable phase shifting means and phase shifting means equipped with twosettings of 0 and phase shifts operative by said control circuit meansto control the phase of said first and second pairs of electromagneticwaves relative to one another,

said variable phase shifting means connecting, in series,

between said condition means and said source of electromagnetic energyand operative to aid in selectively steering said figure-eight patternof radiation, in said second discrete operating state, through selectedazimuthal angles relative to the borehole axis,

said first and second phase shifting means equipped with two settings of0 and 180 phase shifts being connected in series between at least onehelical element of each of said first and second pairs of helicalelements and said source of electromagnetic energy and operative toshift the polarizing mode of said first and second pairs ofelectromagnetic waves propagating along said antenna from a first to asecond polarizing mode.

10. The helical antenna of claim 9 in which said first polarizing modeis vertical and said second polarizing mode is horizontal.

11. The method of irradiating an earth formation penetrated by aborehole with a two-condition, selectively variable, pattern ofelectromagnetic energy whereby the distance and direction of anelectromagnetic discontinuity within said formation, from said borehole,can be indicated and identified, comprising the steps of:

in a transmitting mode, coupling electromagnetic energy from a source ofenergy to N pair(s) of helical conductors and a central cylindricalelement coextensive with said helical conductors at a subset of feedpoints to propagate N pair(s) of electromagnetic waves in coextensiverelationship along and radiating from said conductors into saidformation, each of said waves being specified in terms of voltagesexisting between the helical conductors and the central cylindricalelement and of currents carried by the helical conducting elements andby said cylindrical element, where N is any whole number greater thanzero;

directing said N pair(s) of electromagnetic waves in helical paths ofcommon angular direction about said cylindrical element toward N remoteend(s) of said cylindrical element;

controlling, in a first discrete operating state, the direction ofcurrent phasers of said N pair(s) of electromagnetic waves as a functionof the symmetrical position of said waves as measured along said antennato irradiate electromagnetic energy into said earth formation which isomnidirectional in planes transverse to the axis of the borehole;

in a second discrete operating state, reversing the axial direction oftravel of current phasers of N waves of said N pair(s) of coextensivelypropagating electromagnetic waves and thereafter controlling saidreversed current phasers so as to irradiate into said formation a highlydirectional, in azimuth figure-eight radiation pattern.

12. The method as in claim 11 in which N=l whereby one pair ofelectromagnetic waves is generated between one pair of helicalconducting elements and said central element and, in said first discreteoperating state, propagate along said central element in a common axialdirection, but, in said second discrete operating state, said waves ofsaid one pair of waves propagate in opposite axial directions along saidcentral element.

[3. A method as in claim 11 in which N=2 whereby first and second pairsof electromagnetic waves are generated between first and second pairs ofhelical conducting elements and said central element,

is said first discrete operating state, each pair of electromagneticwaves begin approximately at the midplane of said central element andcoextensively propagate in the same axial direction toward a common endof said central element to generate said omnidirectional transverseradiation pattern in said formation, said first pair propagating in anopposite axial direction as said second pair of waves,

in said second discrete state, an individual electromagnetic wave ofeach of said first and second pairs of electromagnetic waves propagatesin coextensive relationship with another individual wave of the samepair of waves but in an opposite axial direction as said another wave soas to irradiate said formation with said directional, figure-eightpattern of electromagnetic radiation.

14. A method as in claim 11 in which the steps of reversing thedirection of current phasers and then controlling the reversed currentphasers to provide a highly directional, figure-eight pattern ofradiation in azimuth are further characterized by the additional step ofsteering said figure-eight radiation pattern through selected azimuthalangles relative to the axis of borehole to isolate the azimuthaldirection of said discontinuity relative to said borehole axis.

15. A method in accordance with claim 13 with the additional step ofswitching polarizing mode of said first and second pairs ofelectromagnetic waves from a first mode to a second polarizing mode andthereafter comparing signals received from a common discontinuity ineach polarizing mode so as to indicate textural nature of saiddiscontinuity.

1. In a subsurface earth formation exploration tool for logging an earthformation penetrated by a borehole to approximate the distance anddirection to an electromagnetic discontinuity in said formation fromsaid borehole by measuring the two-way travel time of electromagneticenergy generated by means of an energization circuit including a sourceof electromagnetic energy and a control circuit means, uphole orinterior of said tool, a helical antenna supported within said tool forirradiating said formation with a two-condition, selectively variablepattern of electromagnetic radiation, in azimuth, comprising: a. acentral conducting element axially elongated substantially along theaxis of said well bore, b. at least N pair(s) of helical conductingelements wound axially along and radially spaced outwardly from saidcentral element, where N is any whole number greater than zero, eachhelical conducting element of each N pair(s) of elements winding in thesame circumferential direction about said central element toward acommon end of said central element, c. switching coupler meanspositioned interior of said central element and including a plurality offeed points positioned along said helical antenna for coupling, fromsaid energization circuit, electromagnetic energy between said centralelement and said N pair(s) of helical elements, and condition meansoperatively connected to and controlled by said control circuit meansfor coupling, among said plurality of feed points, electromagneticenergy to a first subset of feed points in a first discrete operatingstate so as to produce, along said antenna, at least N coextensivelypropagating pair(s) of electromagnetic waves propagating in phase alongsaid antenna in a common axial direction, said N pair(s) ofelectromagnetic energy radiating from said antenna in phase into saidadjacent earth formation in planes substantially transverse to the axisof said well bore to produce a uniform, omnidirectional transversEradiation pattern, said conditioning means, in a second discreteoperating state, in response to a change in condition within saidcontrol circuit means, sequentially decoupling N adjacent feed point(s)of said subset of feed points from operative contact with saidenergization circuit and then coupling N end feed point(s) to saidenergization circuit whereby N pair(s) of coextensively propagatingelectromagnetic waves are produced, said N pair(s) of electromagneticwaves, in said second discrete operating state, each traveling inopposite axial directions along said antenna but interacting so as toirradiate, into such adjacent earth formation, a highly directional,figure-eight pattern of electromagnetic energy.
 2. The helical antennaof claim 1 in which N 1, whereby one pair of electromagnetic waves isgenerated between one pair of helical conducting elements and saidcentral element, and, in said first discrete operating state, propagatealong said central element in a common axial direction but, in saidsecond discrete operating state, said waves of said one pair of wavespropagate in opposite axial directions along said central element. 3.The helical antenna of claim 1 in which N 2, whereby first and secondpairs of electromagnetic waves are generated between first and secondpairs of helical conducting elements and said central element, in saidfirst discrete operating state, each pair of electromagnetic waves beginsubstantially at the midplane of said central element and coextensivelypropagate in the same axial direction toward a common end of saidcentral element to generate said omnidirectional transverse radiationpattern in said formation, said first pair propagating in an oppositeaxial direction as said second pair of waves, in said second discretestate, an individual first or second electromagnetic wave of each firstand second pairs of waves propagates in coextensive relationship withanother individual wave of the same pair of waves but in an oppositeaxial direction as said another wave so as to generate said directional,figure-eight pattern of electromagnetic radiation.
 4. The helicalantenna of claim 3 with the addition of phase shifting meanselectrically connected to said control circuit means and operatedthereby, in said second discrete operating state, to selectively shiftthe phase of said first and second pairs of electromagnetic wavespropagating between said first and second pairs of helical conductingelements and said central element so as to cause selective steerage, inazimuth, of said figure-eight pattern of radiation relative to the axisof said borehole and thereby aid in identifying the azimuthal directionof said discontinuity relative to said borehole axis.
 5. The helicalantenna of claim 4 with the addition of first and second phase shiftingmeans equipped with two settings of 0* and 180* phase shiftselectrically controlled by said control circuit and operative in saidfirst and second discrete operating settings to shift the polarizingmode of said first and second pairs of electromagnetic waves from afirst to a second polarizing mode.
 6. The helical antenna of claim 3 inwhich said switching coupler is further characterized by a series ofsegmented conductor means interior of said central element, said seriesof conducting means joined together at a series of junctions interior ofsaid central element and terminating in a plurality of terminatingsegments, said first subset of feed points in said first discreteoperating state comprising four centrally disposed feed pointsconnecting, through said terminal segments, said first and second pairsof helical conducting elements and said central element to said sourceof electromagnetic energy, said four central feed points definingcorners of an imaginary parallelogram lying in a plane through saidswitching coupler, one set of adjacent diagonally located feed pointsbeing connected in series with said source of electromagnetic energythrough selected lengths of said segmented conductor means so as tocouple electromagnetic energy to said helical elements in a phaseopposite to that coupled to the remaining set of diagonally located feedpoints.
 7. The helical antenna of claim 6 in which said condition meansis further characterized by first and second switch means each locatedadjacent to a remote end of said central element as measured from themidplane thereof, in said second discrete operating state, said firstand second switch means operating to decouple said one set of adjacentdiagonally located feed points from contact with said source of energyand then couple said source to said end feed points remote from saidmidplane to launch electromagnetic waves along said antenna in anopposite axial direction than in said first discrete operating state. 8.The helical antenna of claim 7 in which first and second switch means,in changing from said first to said second discrete operating states,simultaneously reorient matched loads located at remote ends of saidcentral element to contact the terminal ends of said first and secondpairs of helical conducting elements so as to prevent reflections ofterminating electromagnetic waves back through said antenna toward saidfeed points.
 9. The helical antenna of claim 6 in which said pluralityof terminal segments of said conductor means interior of said centralelement connect, in series, to said source of electromagnetic energythrough a variable phase shifting means and phase shifting meansequipped with two settings of 0* and 180* phase shifts operative by saidcontrol circuit means to control the phase of said first and secondpairs of electromagnetic waves relative to one another, said variablephase shifting means connecting, in series, between said condition meansand said source of electromagnetic energy and operative to aid inselectively steering said figure-eight pattern of radiation, in saidsecond discrete operating state, through selected azimuthal anglesrelative to the borehole axis, said first and second phase shiftingmeans equipped with two settings of 0* and 180* phase shifts beingconnected in series between at least one helical element of each of saidfirst and second pairs of helical elements and said source ofelectromagnetic energy and operative to shift the polarizing mode ofsaid first and second pairs of electromagnetic waves propagating alongsaid antenna from a first to a second polarizing mode.
 10. The helicalantenna of claim 9 in which said first polarizing mode is vertical andsaid second polarizing mode is horizontal.
 11. The method of irradiatingan earth formation penetrated by a borehole with a two-condition,selectively variable, pattern of electromagnetic energy whereby thedistance and direction of an electromagnetic discontinuity within saidformation, from said borehole, can be indicated and identified,comprising the steps of: in a transmitting mode, couplingelectromagnetic energy from a source of energy to N pair(s) of helicalconductors and a central cylindrical element coextensive with saidhelical conductors at a subset of feed points to propagate N pair(s) ofelectromagnetic waves in coextensive relationship along and radiatingfrom said conductors into said formation, each of said waves beingspecified in terms of voltages existing between the helical conductorsand the central cylindrical element and of currents carried by thehelical conducting elements and by said cylindrical element, where N isany whole number greater than zero; directing said N pair(s) ofelectromagnetic waves in helical paths of common angular direction aboutsaid cylindrical element toward N remote end(s) of said cylindricalelement; controlling, in a first discrete operating state, the directionof current phasers of said N pair(s) of electromagnetic waves as afunction of the symmetrical poSition of said waves as measured alongsaid antenna to irradiate electromagnetic energy into said earthformation which is omnidirectional in planes transverse to the axis ofthe borehole; in a second discrete operating state, reversing the axialdirection of travel of current phasers of N waves of said N pair(s) ofcoextensively propagating electromagnetic waves and thereaftercontrolling said reversed current phasers so as to irradiate into saidformation a highly directional, in azimuth figure-eight radiationpattern.
 12. The method as in claim 11 in which N 1 whereby one pair ofelectromagnetic waves is generated between one pair of helicalconducting elements and said central element and, in said first discreteoperating state, propagate along said central element in a common axialdirection, but, in said second discrete operating state, said waves ofsaid one pair of waves propagate in opposite axial directions along saidcentral element.
 13. A method as in claim 11 in which N 2 whereby firstand second pairs of electromagnetic waves are generated between firstand second pairs of helical conducting elements and said centralelement, is said first discrete operating state, each pair ofelectromagnetic waves begin approximately at the midplane of saidcentral element and coextensively propagate in the same axial directiontoward a common end of said central element to generate saidomnidirectional transverse radiation pattern in said formation, saidfirst pair propagating in an opposite axial direction as said secondpair of waves, in said second discrete state, an individualelectromagnetic wave of each of said first and second pairs ofelectromagnetic waves propagates in coextensive relationship withanother individual wave of the same pair of waves but in an oppositeaxial direction as said another wave so as to irradiate said formationwith said directional, figure-eight pattern of electromagneticradiation.
 14. A method as in claim 11 in which the steps of reversingthe direction of current phasers and then controlling the reversedcurrent phasers to provide a highly directional, figure-eight pattern ofradiation in azimuth are further characterized by the additional step ofsteering said figure-eight radiation pattern through selected azimuthalangles relative to the axis of borehole to isolate the azimuthaldirection of said discontinuity relative to said borehole axis.
 15. Amethod in accordance with claim 13 with the additional step of switchingpolarizing mode of said first and second pairs of electromagnetic wavesfrom a first mode to a second polarizing mode and thereafter comparingsignals received from a common discontinuity in each polarizing mode soas to indicate textural nature of said discontinuity.